An active site of an enzyme is a____into which of the substrate fits.

Answer

### Step-by-Step Solution:

1. **Understanding the Active Site**: The active site of an enzyme is a specific region where the substrate binds. It is crucial for the enzyme's catalytic activity.

2. **Visualizing the Structure**: Imagine the enzyme as a three-dimensional structure with a particular shape. The active site is a small part of this structure, often described as a pocket or groove.

3. **Function of the Active Site**: The active site contains the functional groups that facilitate the conversion of the substrate into a product. When the substrate enters the active site, it can undergo a chemical reaction.

4. **Substrate Binding**: The substrate fits into the active site, much like a key fits into a lock. This interaction is specific, meaning that each enzyme typically binds to a particular substrate.

5. **Formation of Enzyme-Substrate Complex**: Once the substrate is bound to the active site, an enzyme-substrate complex is formed, which leads to the transformation of the substrate into the product.

6. **Release of Product**: After the reaction, the product is released, and the enzyme is free to bind to another substrate molecule.

7. **Final Answer**: Based on the above understanding, we can conclude that the active site of an enzyme is a **pocket** into which the substrate fits.

### Final Answer:
An active site of an enzyme is a **pocket** into which the substrate fits.

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